Previously when loading a YAML config, it overrode single values on the second hierarchy level.
But on all deeper levels new values were completely overwritten. This was inconsistent and poses
a problem for config entries that use these levels, like the ca-trust or the in-development autoterminal.
This change implements full merging of dictionaries on all levels to solve this. Values in files later
in precedence overwrite previous values. If the the old and the new config values are both dictionaries,
then the hierarchy levels are merged down recursively.
You can remove a previously existing dict entry again in a later file by assigning it an empty value.
Also fix handling of invalid YAMLs: ignore them instead of aborting execution
